The present invention relates to a method and apparatus for transforming a real digital wideband high-frequency signal into a set of complex digital baseband signals, a method and apparatus for transforming a set of complex digital baseband signals into a real digital wideband high-frequency signal, a preferred type of complex filter that may be used in these transformations and a base station in a radio communication system that uses these transformations.
A base station in a mobile telephony system receives and transmits wideband high-frequency radio signals having a bandwidth of up to 30 MHz. The received wideband signal is separated into narrowband (for example 30 kHz wide) channels (FDMA) or channel groups (TDMA). In the reverse process channels or channel groups are combined into a wideband signal for transmission.
It has been suggested to perform A/D conversion of the entire wideband spectrum and to perform the channel separation digitally. For each channel or channel group the digital wideband signal is demodulated with a different frequency in order to shift this particular channel or channel group down to the baseband. The demodulation is performed with a quadrature network for generating the I and Q components. Thereafter these components are low-pass filtered in order to separate the desired channel or channel group from the unwanted neighbor channels or channel groups. Finally the samples of the separated signals are decimated (down-sampled).
In the reverse process baseband signals are interpolated (up-sampled), modulated and combined into a wideband signal.
A drawback of these methods is that the demodulation and modulation has to be performed at the high sampling frequency of the digital wideband signal, which requires a lot of data processing. Furthermore, the required local oscillators and multipliers require a lot of space and are power consuming.
An object of the present invention is to reduce the amount of required data processing in the transformation from wideband signal to baseband signals and from baseband signals to wideband signals.
This object is solved by the method, apparatus and base station in accordance with the accompanying claims.
A further object of the present invention is a preferred type of complex filter that is used in this transformation.
Briefly, the present invention performs the channel separation at the high wideband signal sampling frequency by using complex bandpass filters. Instead of demodulating the wideband signal down to baseband, a frequency reduction may be performed on the I and Q signals that are obtained directly from the complex filters simply by decimating (down-sampling) the number of samples.
In a modification of the present invention, used when the baseband may not reached directly by decimation, most of the advantage may still be obtained by performing a demodulation to baseband after decimation to a frequency near baseband.
Similarly, a complex filter may be used to filter an interpolated (up-sampled) baseband or low frequency signal for obtaining a narrowband high-frequency signal. Such high-frequency signals may then be combined into a wideband signal for transmission.
In a modification of the present invention, used when the high frequency band may not reached directly by interpolation, most of the advantage may still be obtained by performing a modulation to a frequency near baseband before the interpolation.